Through-the-lens illuminator for optical comparator

ABSTRACT

An illumination system shares portions of an objective of an optical inspection system. A plurality of beam-shaping optics collects light from a plurality of effective light sources and directs the light through a portion of the objective for illuminating an object under inspection. The objective includes a front relay lens, a rear relay lens, and an objective stop disposed between the front and rear relay lenses for collecting light scattered from the object and forming an image of the object with the collected light. The beam-shaping optics, which surround the objective stop, are arranged together with the associated effective light sources for non-uniformly distributing light within a range of angles required for illuminating the object.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/948,903, filed Nov. 18, 2010, which application is herebyincorporated by reference.

TECHNICAL FIELD

The invention arises within the field of metrology as an improvement toillumination systems for optical comparators and relates particularly toillumination systems that share optical paths with imaging systems ofoptical inspection machines.

BACKGROUND OF THE INVENTION

Optical comparators project images of objects under inspection ontodisplay screens for comparison against a reference datum. Comparisons tothe reference datum can be made in association with different types ofillumination, including direct lighting, back lighting, and obliquelighting of the objects.

The comparators include optical imaging systems responsible forprojecting the images of objects under inspection onto display screens.Objectives can be used to form intermediate images of the objects, andthe intermediate images can be magnified by projectors for producingdisplay images capable of comparison to the reference datum.

So called “through-the-lens” illuminating systems have been used forilluminating the objects under inspection by directing light through theobjectives to the objects. The illuminating light is generally producedby light sources located remote from the optical imaging systems butproducing light beams that generally intersect light paths through theoptical imaging system. Inclined mirrors of such illuminating systemssurrounding aperture stops of the illuminating systems fold theilluminating beams into alignment with the light paths of the imagingsystems.

Light sources of the type used for such illuminating systems, such asmercury arc lamps, tend to be large and expensive, and can raise safetyconcerns. Some countries, for example, have banned mercury arc lamps.The invention includes among its objects the replacement of such largelight sources while providing through-the-lens illumination of objectsunder inspection.

SUMMARY OF THE INVENTION

The invention, as may be practiced within certain preferred embodimentsof through-the-lens illumination systems, replaces large single lightsources located remote from the shared pathways of coextending portionsof imaging systems with a plurality of smaller light sources andassociated beam-shaping optics located coaxially with the sharedpathways. The plurality of light sources can include light-emittingdiodes (LEDs) whose output is shaped by the beam-shaping optics forfilling portions of an effective illuminator aperture surrounding anaperture stop of an objective of the imaging system. Althoughpropagating in opposite directions, both the imaging light filling theobjective stop and the illuminating light filling portions of asurrounding space can pass through a common optical element of theobjective for directing light both to and from an object underinspection. Light distributions from the beam-shaping optics arepreferably arranged for more efficiently conveying generated light tothe object under inspection and for more uniformly distributing thelight over the object field.

One example of an optical inspection system arranged in accordance withthe invention includes both an illuminating system for illuminating anobject under inspection and an imaging system for imaging the objectunder inspection within an object field. An objective is shared in partby both the illuminating system and the imaging system and includes afront relay lens, a rear relay lens, and an objective stop disposedbetween the front and rear relay lenses. The objective, which ispreferably at least approximately telecentric, collects light scatteredfrom the object and forms an image of the object with the collectedlight. The image can be a final image but is preferably in intermediateimage that is projected onto a comparator screen. A multiplexed beamgenerator of the illuminating system includes a plurality of effectivelight sources and associated beam-shaping optics surrounding theobjective stop for illuminating the object. Each of the beam-shapingoptics is arranged together with its associated effective light sourcefor non-uniformly distributing light within a range of angles requiredfor illuminating the object field.

Each of the beam-shaping optics and its associated effective lightsource is preferably arranged for distributing light differently withinthe range of required angles so that the object field is moreefficiently and uniformly illuminated. For example, the beam-shapingoptics can be distributed in pairs symmetrically about an optical axisof the objective and the non-uniform distributions of light within thepairs can be substantially mirror symmetrical. The range of anglesthrough which light is distributed from the beam-shaping optics caninclude angles at which the light both converges toward and divergesfrom an optical axis of the objective and the beam-shaping opticspreferably distribute more light into the angles that converge towardthe optical axis.

The effective light sources are preferably relatively sized forproducing together with the beam-shaping optics a limited range ofangularly related light beams that are converted by the front relay lensinto a range of spatially distributed beams over the object field. Thebeam-shaping optics include optical axes, and in one arrangement, theassociated effective light sources can have centers that are offset fromthe optical axes of the beam-shaping optics. Preferably, the centers ofthe effective light sources are offset from the optical axes of thebeam-shaping optics in directions that extend radially of an opticalaxis of the objective.

In another arrangement, the beam-shaping optics include optical axesthat are inclined to an optical axis of the objective. The axes of thebeam-shaping optics are preferably inclined in axial planes containingthe axis of the objective.

For cost considerations, the relay lenses of the objective preferablyhave a limited size. The numerical aperture of the objective through thefront relay lens is preferably limited by the objective stop inaccordance with the general requirements for approaching telecentricity.The illumination system operates through the front relay lens of theobjective at a higher numerical aperture, and as such, only theilluminating light closest radially to the objective stop approaches theobject field as near telecentric light. Illuminating light fartherradially from the objective stop progressively departs fromtelecentricity. For example, an aperture of the front relay lens canblock angles that would otherwise have contributed to more telecentricillumination.

In another arrangement, the beam-shaping optics can have peripheriesthat are truncated adjacent to the objective stop. The optical axes ofthe beam-shaping optics are positioned closer to the objective stopthrough radial distances that approximately correspond to amounts thatthe peripheries of the beam-shaping optics are truncated in commonradial directions. With the illuminating light concentrated closerradially to the objective stop, angular uniformity at the object fieldcan be increased along with spatial uniformity and overall efficiency.Spatial uniformity can be increased at some cost to angular uniformityby increasing the distribution of light among certain of the angles thatconverge toward the optical axis of the objective.

An illumination system in accordance with another example of theinvention is particularly adapted for use with an optical inspectionsystem having an objective for forming an image of an object underinspection. A plurality of effective light sources and associatedbeam-shaping optics are arranged for collecting light from the pluralityof light sources and directing the light through a portion of theobjective for illuminating the object. A common housing for thebeam-shaping optics has a central aperture about which the beam-shapingoptics are mounted and within which a stop of the objective is defined.An optically transmissive plate located within the central aperture ofthe common housing blocks transmissions of heat through the objectivestop.

Locating the plurality of effective light sources proximate to theimaging pathway can produce heat disturbances within the imaging system(e.g., heat waves) particularly if one or the other of the relay lensesis located along a convection pathway. Locating the opticallytransmissive plate within the central aperture of the common housingblocks the propagation of heat along the imaging pathway. The opticallytransmissive plate is preferably inclined out of a normal orientation toan optical axis of the imaging system to avoid producing spurious imagesof the object. A seal between the optically transmissive plate and thecommon housing prevents even minor air flow through the housing'scentral aperture. In addition, the optically transmissive plate ispreferably thermally coupled to the common housing for cooling theplate. The common housing itself can be actively cooled, such as byflowing coolant through or around the housing.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a diagram of an optical comparator showing imaging pathways ofan imaging system from an object under inspection to a screen forcomparing the object under inspection to a datum.

FIG. 2 is a diagram of a portion of the imaging system shared by anillumination system for illuminating the object under inspection showingillumination pathways from a multiplexed beam generator shown in crosssection to the object.

FIG. 3 is a relatively enlarged axial view of the multiplexed beamgenerator showing a distribution of beam-shaping optics surrounding anaperture stop of the imaging system.

FIG. 4 is a diagram of an alternative illumination system for theoptical comparator in which effective light sources are radially offsetwith respect to axes of beam-shaping optics within a modifiedmultiplexed beam generator.

FIG. 5 is a diagram of another alternative illumination system for theoptical comparator in which beam-shaping optics together with theirassociated effective light sources within a modified multiplexed beamgenerator are inclined toward a common axis of the imaging andillumination systems.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, an optical comparator 10, as an example of anoptical inspection system in accordance with the invention, includes anobjective 12 for forming an intermediate image 16 of an object 14 underinspection and a projector 18 for forming a magnified image 20 of theobject 14 on a screen 22. The magnified image 20 can be compared to adatum, such as a template, for checking if the object 14 is withincertain tolerances or taking other measurements. While the objective 12can be regarded as an imaging system through the formation of theintermediate image 16, in a larger sense, the objective 12 combined withthe projector 18, which together form the magnified image 20,constitutes a more complete imaging system of the comparator 10.Although not shown, the comparator 10 can include other conventionalfeatures (not shown) including one or more stages for mounting andmoving the object 14, other illumination systems for bright-light orback-light illumination, gaging apparatus for measuring or otherwisecomparing the object to a datum, and controls for the various comparatorfunctions and for linking the comparator to other systems such asprocessing, work flow, or communications systems.

The objective 12 includes a front relay lens 24 and a rear relay lens 28straddling an aperture stop 26, which is preferably located at the backfocal plane of the front relay lens 24 and at the front focal plane ofthe rear relay lens 28. The front relay lens 24 collects light from anobject field 30 in which the object 14 is located and the rear relaylens 28 forms the intermediate image 16 of the object 14 within anintermediate image field 32. The aperture stop 26 preferably constrainsa range of field angles collected form the object field 30 so that theobjective 12 is at least approximately telecentric for avoidingdistortions of the intermediate image 16 accompanying variations in thefocal depth of the object 14 throughout the object field 30. Preferably,the objective 12 has a one-to-one magnification and is doublytelecentric, i.e., telecentric at both the object field 30 and theintermediate image field 32.

The projector 18, which can provide various amounts of magnification,projects the intermediate image 16 onto the comparator screen 22. Foldmirror 34 within the objective 12 and fold mirror 36 between theprojector 18 and screen 22 exemplify a redirection of imaging lightwithin the comparator 10 for interconnecting locations convenient formounting the object 14 and observing the magnified image 20 of theobject 14.

A through-the-lens illumination system 38 in accordance with theinvention includes a multiplexed beam generator 40 surrounding theobjective aperture stop 26. The illustrated multiplexed beam generator40, which is also shown by FIG. 2 and further enlarged in FIG. 3,includes four effective light sources 44 and associated fourbeam-shaping optics 46 all mounted within a common housing 50. A centralaperture within the common housing 50 functions as the stop 26 of theobjective 12. The exit faces of the beam-shaping optics 46 arepreferably located in or near a plane containing the objective stop 26so that the output of the beam-shaping optics 46 fills at least portionsof an effective illuminator aperture stop 58 within the same plane. Thefour effective light sources 44 preferably comprise an equal number oflight-emitting diodes (LEDs). The beam-shaping optics 46 preferablycomprise collector or collimating lenses for converting output from theeffective light sources 44 into an angular spread of beams forcollectively illuminating the object field 30 through the front relaylens 24 of the objective 12.

The effective light sources 44 can be shaped and sized for achieving thedesired angular spread of light beams through the beam-shaping optics 46by combining the LEDs with respective diffusers (not shown) between theLEDs and collimating lenses. In the embodiment shown, the effectivelight sources 44 are aligned with axes 48 of the beam-shaping optic 46,and the axes of the beam-shaping optics are evenly distributed around anaxis 42 of the objective 12 (which axis is also common to theillumination system 38). Preferably, the beam-shaping optics 46 arearranged in mirror symmetrical pairs for delivering balanced angulardistributions of light across the object field 30.

Although shown as four (e.g., two pairs) of beam-shaping optics 46, moreor less beam-shaping optics and associated light sources can be used(preferably in mirror symmetrical pairs). The beam-shaping opticspreferably fill as much as possible of an annular periphery surroundingthe aperture stop and are limited in overall radial dimension inaccordance with a desired diameter of an effective illuminator aperturestop setting the maximum field angle for illuminating the object field30.

As also shown in the illustrations of FIGS. 2 and 3, the beam-shapingoptics 46 have peripheries that are truncated (see flats 54) adjacent tothe objective stop 26, which allows the axes 48 of the beam-shapingoptics 46 to be moved radially closer to the stop 26. The radial movesof the axes 48 through the amount the optics 46 are radially truncatedallows more light from the beam-shaping optics 46 to pass through thefront relay lens 24 while concentrating light within field angles closerto the field angles collected by the objective 12 for forming theintermediate image 32. The radially truncated form of the beam-shapingoptics 46 effectively flattens a portion of an otherwise circularperiphery, but other truncated shapes could also be used includingconcave shapes to more closely match the form of the objective stop 26.

The effective light sources 44 and associated beam-shaping optics 46surrounding the objective stop 26 are referred to as a multiplexed beamgenerator because the spatial and angular contributions from each of thelight sources 44 and associated beam-shaping optics 46 combine toilluminate at least partially overlapping portions of the object field30 over a spread of field angles. The front relay lens 24 operatingthrough a first effective numerical aperture collects light from theobject field 30 through a range of field angles limited by the size ofthe objective stop 26. As a part of the illuminating system 38, thefront relay lens 24 operates through a higher numerical aperture notlimited by the objective stop 26 for conveying light to the object field30 through a higher range of angles, which can be limited by either theaperture of the front relay lens 24 or the effective illuminatoraperture 58 encompassing the beam-shaping optics 46. The truncated form(e.g., flats 54) of the beam-shaping optics 46, which moves the axes 48of the beam-shaping optics 46 closer to the objective stop 26, allowsmore light to pass through the front relay lens 24 and concentrates thelight within field angles that reach more of the object field 30.

The common housing 50 and its adaptations also reduce potentiallyundesirable effects from heat generated by the effective light sources44 close to the imaging pathway of the comparator 10. For purposes offurther isolating the effective light sources 44 and blocking heatconvection along the optical axis 42 toward the front relay lens 24, anoptically transmissive plate 52 is located within the central apertureof the housing 50 covering the aperture stop 26. Preferably, theoptically transmissive plate 52, which can be made of optical glass, ismounted slightly tipped (e.g., the optical axis of the transmissiveplate 52 is inclined to the objective axis 42 by approximately 10degrees) to avoid producing spurious reflections along the imagingpathway from the object field 30 to the intermediate image field 32.Anti-reflective coatings can be applied to the plate to reducereflections and enhance transmissivity. The plate 52 is preferablysealed to the central aperture of the common housing 50. Conventionalsealing materials can be used for this purpose, such as room-temperaturevulcanizing (RTV) silicones. The sealing material preferably (a) blocksheat flows around the plate 52, (b) provides a thermally conductivepathway between the plate 52 and the housing 50 to uniformizetemperatures, and (c) provides a secure and steady mounting for theplate within the housing 50. The effective light sources 44, which arealso mounted within the common housing 50, tend to transfer heat intothe housing 50. A circulating cooling system 60 connected to the commonhousing 50 extracts the excess heat from the housing 50. For example, acoolant such as water can be circulated between the housing 50 and aheat exchanger. A fan (not shown) can also be used for conveying heatfrom the light sources 44.

An alternative multiplexed beam generator 70 is shown in FIG. 4. Similarto the multiplexed beam generator 40, the multiplexed beam generator 70includes a plurality of effective light sources 74 and associatedbeam-shaping optics 76 arranged within a common housing 80 in mirrorsymmetrical pairs (as is generally preferred) around the aperture stop26 of the objective 12. However, instead of truncating the peripheriesof the beam-shaping optics 76, the effective light sources 74 areradially displaced from axes 78 of the beam-shaping optics 76.Centerlines 82 of the effective sources are displaced radially outwardlyfrom the axes 78 of the beam-shaping optics 76 through the distance dR.

The radial offset dR of the effective sources 74 concentrates lightwithin a range of aperture angles (within the illumination aperture)prone to reaching the object field 30 through the front relay lens 24.The mirror symmetry between the effective sources 74 and associatedbeam-shaping optics 76 on opposite sides of the objective stop 26provides for balancing illumination across the object field 30. That is,the object field positions disfavored by the effective sources 74 andassociated beam-shaping optics 76 on one side of the objective stop 26are favored by the mirror symmetrical effective sources 74 andassociated beam-shaping optics 76 on the opposite side of the objectivestop 26. The shapes and sizes of the effective light sources 74 arepreferably limited in relation to the beam-shaping optics 76 forgenerating light beams through the range of aperture angles required forilluminating the object field 30 as imaged by the objective 12 or theprojector 18. Preferably, the radial offset dR is limited to relativelyincreasing concentrations of light within certain of the aperture angleswithin the range required for illuminating the object field 30.

Another alternative multiplexed beam generator 90 is shown in FIG. 5.Here, a plurality of effective light sources 94 remain aligned with axes98 of associated beam-shaping optics 96 within a common housing 100, butthe axes 98 of the beam-shaping optics 96 are inclined to the objectiveoptical axis 42 through an angular offset dθ (shown measured withrespect to parallel axes 102) for concentrating light within apertureangles prone to reaching the object field 30 through the front relaylens 24. The inclinations of the axes 98 are preferably taken withinaxial planes that include the objective axis 42. Similar to thepreceding embodiment, mirror symmetry is preferably provided between theeffective sources 94 and associated beam-shaping optics 96 on oppositesides of the objective stop 26 for balancing illumination across theobject field 30. Also similar to the preceding embodiment, the shapesand sizes of the effective light sources 94 are preferably limited inrelation to the beam-shaping optics 96 for generating light beamsthrough the range of aperture angles required for illuminating theobject field 30 as imaged by the objective 12 or the projector 18. Theangular offset dθ is limited to relatively increasing concentrations oflight within certain of the aperture angles within the range requiredfor illuminating the object field 30.

Various combinations of offsetting the effective light sources andinclining the beam-shaping optics can be used along with optimallysizing and shaping of the effective light sources to controldistributions of light among aperture angles required for illuminatingthe imaged object field 30. In addition, the peripheries of thebeam-shaping optics can be truncated to shift the inclined or relativelyoffset axes of the beam-shaping optics closer to the objective aperturefor concentrating light within field angles that are closer to the fieldangles at which the object field 30 is imaged.

Although described with respect to an optical comparator, as an exampleof the invention's preferred use, the teachings of this invention areexpected to apply in general to optical inspection systems with combinedimaging and illumination systems, particularly where the minimumnumerical aperture through which the illuminator operates is beyond thenumerical aperture within which the imager operates.

1. An illumination system for an optical inspection system including anobjective for forming an image of an object under inspection comprising:a plurality of effective light sources and associated beam-shapingoptics that are arranged for collecting light from the plurality ofeffective light sources and directing the light through a portion of theobjective for illuminating the object; a common housing for thebeam-shaping optics having a central aperture about which thebeam-shaping optics are mounted and within which a stop of the objectiveis defined; and an optically transmissive plate located within thecentral aperture of the common housing for blocking transmissions ofheat through the objective stop.
 2. The illumination system of claim 1in which the optically transmissive plate is inclined to avoid producingspurious images of the object.
 3. The illumination system of claim 2 inwhich the optically transmissive plate has an optical axis that isinclined with respect to an optical axis of the objective byapproximately 10 degrees.
 4. The illumination system of claim 1 furthercomprising a seal between the optically transmissive plate and thecommon housing.
 5. The illumination system of claim 1 in which theoptically transmissive plate is thermally coupled to the common housingfor cooling the plate.
 6. The illumination system of claim 1 in whichthe common housing is actively cooled.
 7. The illumination system ofclaim 1 in which the objective includes front and rear lenses straddlingthe objective stop, the front lens is arranged for collecting light fromthe object, and the plurality of effective light sources include lightemitting diodes (LEDs) that are supported by the common housing inpositions between the objective stop and the rear lens.
 8. Theillumination system of claim 7 in which a beam shaping optic isassociated with each of the plurality of effective light sources.
 9. Theillumination system of claim 8 in which (a) the objective collects lightscattered from the object through a first range of field angles and theplurality of effective light sources and associated beam-shaping opticsare located in positions surrounding the objective stop for illuminatingthe object through a second range of higher field angles, and (b) theplurality of effective light sources and associated beam-shaping opticsconcentrate light among angles within the second range of field anglesthat are closer to angles within the first range of field angles forreducing angular variation among the field points illuminating theobject.
 10. The illumination system of claim 9 in which the second rangeof angles through which light is distributed from the beam-shapingoptics includes angles at which the light both converges toward anddiverges from an optical axis of the objective and the beam-shapingoptics distribute more light into the angles that converge toward theoptical axis.